This invention relates to lighting panels of the sort used in distributing light from a light source. Prismatic lighting panels are widely used in overhead fluorescent lighting fixtures, and may be used with other light sources. Their primary purpose is to reduce direct glare by controlling the angle at which light emerges from the panel.
The theory of prismatic lighting panels is well known, and is discussed, for example, in McPhail, U.S. Pat. No. 2,474,317. Such panels include a planar upper face and a lower face covered with prismatic elements. Light rays entering the top of the panel are either refracted downward through the lower surface of the panel at useful angles to the vertical (i.e. the normal of the panel), or are reflected internally by the prismatic elements upward through the upper surface of the panel. If the prismatic elements have straight sides which make the proper angle with the normal of the panel, virtually all of the light which would otherwise emerge at high angles relative to the normal of the panel is internally reflected by the prisms and high angle "direct" glare is thereby greatly reduced or eliminated.
A particularly popular prismatic lighting panel has, on its lower surface, female conical prisms, the apexes of which are aligned along 45.degree. diagonals to the edges of the panel and spaced three-sixteenths of an inch (0.50+/-0.05 centimeters) on centers. The intersections of the cones thus form a structure of square cells, all of whose sides lie along lattice lines running at angles of 45.degree. to the edges of the panel. An example of such a lighting panel is one sold by K-S-H, Inc., under the trademark KSH-12. For convenience, a panel having this pattern will be referred to herein as "the usual" panel.
In recent years there has been an increasing demand for inexpensive prismatic lighting panels. Because the plastic material of which the panels are made represents the major cost of prismatic lighting panels, the usual prismatic panel has been made ever thinner, until presently it has reached the limit permitted by its geometry. Because the apex angle of the female prisms is critical to the optical performance of the panel, the height of the prisms is a function of the size of the individual cells of the prismatic pattern, that is, the spacings between apexes of the cones. Although it is theoretically possible to reduce the size of the cells below the three-sixteenths inch side of the usual panel, both aesthetic considerations and manufacturing constraints have made any substantial decrease in cell size impractical. More importantly, further reduction in cell size and panel thickness produces such a flimsy panel that it sags noticeably in a standard two-foot by four-foot lighting fixture. Other techniques for reducing the amount of material in a panel have been attempted, such as increasing the apex angle of the female cones or "hogging out" the prisms so that their surfaces are concave rather than linear in cross section, but these techniques yield only limited savings of material and tend to degrade the optical performance of the panel. Using presently known techniques, commercially available embodiments of the usual panel have been reduced to an overall thickness of about 0.093 inch (0.236 centimeters), and have been made as light as 5.4 ounces per square foot (1650 grams per square meter.)